1. Field of the Invention
The present invention relates to an eye examining and testing instrument such as eye refractometer, retinoscope or eye fundus camera.
2. Description of the Prior Art
In such instrument a proper alignment or spacing with respect to the human eye is important for exact measurement or observation or for satisfactory photographing, since an incorrect alignment will result in erroneous measurement where a refractometer is used, or in a partial shielding of the photographing light beam by the iris of the eye where an eye fundus camera is used. On the other hand the spacing is usually predetermined structurally by the distance between the objective lens of the instrument and a support for stabilizing the face of the person to be inspected, but in case of the eye fundus camera the fundus illuminating light will be partly reflected by the cornea and mixed into the photographing light beam to form a flare in the image unless the working distance between the human eye and the objective lens is precisely adjusted. Also an exact spacing allows improvement in the accuracy of the instrument such as a refractometer.
In the comparison of aligning and spacing, an aligning process is rendered possible in many cases even with conventional instruments. For example in case of the eye fundus camera, the alignment is achieved by retracting the camera body from the human eye to focus the objective lens on the anterior portion of the eye, then adjusting the position of camera body in the vertical and horizontal directions in such a manner that the center of eye pupil coincides with the center of the viewing field of the finder and again returning the camera body to the original position. On the other hand an exact spacing has been very difficult to achieve.
U.S. Pat. No. 3,871,772 discloses a method of positioning of an eye examining instrument wherein the anterior chamber of the eye is uniformly illuminated by infrared light through the objective lens and is observed by a sight affixed on the instrument, and the adjustment of alignment and spacing is achieved by matching the center of the pupil with the center of a sight target plate. In this method, however, it is difficult to separate the aligning from the spacing.